Referring to FIG. 5, a conventional I/O connection system 300 will be described. The I/O connection system 300 is composed of hosts 1a and 1b, an Ethernet (registered trademark) switch 3, I/Os 6a and 6b, upstream PCI Express-Ethernet (registered trademark) bridges 7a and 7b (hereafter, referred to as upstream bridges 7a and 7b), a system manager 8, and downstream bridges 9a and 9b. 
The host 1a has a CPU 15a, a memory 17a, a north bridge 16a for interconnecting the CPU 15a, the memory 17a and the upstream bridge 7a, a hot-plug processing unit 11a loaded in the CPU 15a as software to hot-plug the I/O 6a to the host 1a, and a hot-remove processing unit 12a loaded in the CPU 15a as software to hot-remove the I/Os 6a and 6b from the host 1a. The host 1b has a similar configuration.
The switch 3 interconnects the upstream bridges 7a and 7b and the downstream bridges 9a and 9b. 
The I/Os 6a and 6b are assigned to the hosts 1a and 1b in response to their requests.
The upstream bridge 7a connects the host 1a and the Ethernet (registered trademark). The upstream bridge 7b connects the host 1b and the Ethernet (registered trademark).
The system manager 8 sets up connection between the upstream bridges 7a and 7b and the downstream bridges 9a and 9b. 
The downstream bridge 9a connects the I/O 6a and the Ethernet (registered trademark). The downstream bridge 9b connects the I/O 6b and the Ethernet (registered trademark).
Next, description will be made of operation to enable the hosts to use the I/Os in the I/O connection system 300. Herein, description will be made of operation to enable the host 1a to use the I/O 6a by hot-plugging.
In response to input from a system administrator or control from a system management software, the system manager 8 uses a control frame to set up connection of the downstream bridge 9a to the upstream bridge 7a. As the result of this set-up of connection, the I/O 6a connected to the downstream bridge 9a is assigned to the host 1a connected to the upstream bridge 7a. 
Upon receiving the control frame, the downstream bridge 9a requests an interruption to the CPU 15a of the host 1a to notify that the I/O 6a has been assigned to the newly connected host 1a. This interruption invokes the hot-plug processing unit 11a, and hot-plug processing is started.
In the hot-plug processing, the I/O 6a assigned to the host 1a is searched for, a device driver corresponding to the I/O 6a is loaded, and the assigned I/O 6a is registered in connection I/O information held by the host 1a. 
The foregoing processing makes the assigned I/O 6a usable by the host 1a. The interruption to the CPU 15a performed by the downstream bridge 9a may be performed by the upstream bridge 7a to which the host 1a assigned with the I/O 6a is connected.
Next, description will be made of operation of the I/O connection system 300 when an I/O currently connected to a host is hot-removed. Herein, the description will be made of an example in which the I/O 6a connected to the host 1a is hot-removed and connected to the host 1b. 
The upstream bridge 7a detects that the downstream bridge 9a connected thereto is connected to another upstream bridge 7b on the basis of a control packet from the system manager 8 and requests an interruption to the CPU 15a. This interruption invokes the hot-remove processing unit 12a and hot-remove processing is started.
In the hot-remove processing, the I/O 6a released from the assignment to the host 1a is searched for, the device driver corresponding to the I/O 6a is unloaded, and the I/O 6a released from the assignment is deleted from the connection I/O information held by the host 1a. 
Japanese Laid-Open Patent Publication No. 2007-219873 is an example of prior art documents describing an invention relating to this invention.